1. Field of the Invention
The present invention relates to user-programmable circuits. More particularly, the present invention relates to a user-programmable resistor.
2. The Prior Art
Programmable resistances of several varieties are known in the integrated circuit art. According to one variety, a laser-trimmable resistance element is disposed on an integrated circuit. After fabrication of the integrated circuit, but prior to packaging the integrated circuit, the resistance value of the laser-trimmable resistance element may be trimmed using a laser to alter the geometry of the resistance element.
This method has proved to be successful, but its use is limited to a production environment equipped with expensive and complex laser-trimming apparatus. In addition, after the resistance value of the laser-trimmable resistance element has been adusted, the integrated circuit containing it must be subjected to further processing in order to complete the packaging operation that seals the integrated circuit from the environment. Trimming of the resistance value of the laser-trimmable resistance element cannot be performed by the end user of the integrated circuit.
It is also known to use the channel resistance of a floating gate transistor as a variable resistance element. Programming the resistance value is achieved by transferring charge on to or off of the floating gate using one of the available charge transport mechanisms built into the integrated circuit.
There continues to exist a need for alternative solutions to providing a resistance element in an integrated circuit whose resistance value can be programmed by an end user of the integrated circuit.
According to the present invention, a user-programmable resistor module includes a resistive element connected in series with first and second antifuses between an input circuit node and an output circuit node. Third and fourth antifuses are connected in series between the input circuit node and the output circuit node. A first programming transistor is connected between the common connection of the resistive element and the first antifuse and a first programming voltage node. A second programming transistor is connected between the common connection of the first and second antifuses and a fixed voltage node such as ground. A third programming transistor is connected between the input circuit node and the first programming voltage node. A fourth programming transistor is connected between the common connection of the third and fourth antifuses and a fixed voltage node such as ground.
If the user-programmable resistor module described herein is used alone, a termination programming transistor is connected between the output circuit node and a second programming voltage node. However, according to the present invention, a plurality of the user-programmable resistor modules of the present invention may be connected in series to allow programming a plurality of resistance values.
In an arrangement containing a plurality of series-connected user-programmable resistor modules according to the present invention, the input circuit node of each successive user-programmable resistor modules is connected to the output circuit node of the preceding user-programmable resistor module. The connections described above for the second and fourth programming transistors in all additional series-connected user-programmable resistor modules are the same for all user-programmable resistor modules.
The first programming transistor of each odd numbered user-programmable resistor module is connected between the common connection of its resistive element and its first antifuse and the first programming voltage node as described for the individual user-programmable resistor module described above. The first programming transistor of each even numbered user-programmable resistor module is connected between the common connection of its resistive element and its first antifuse and the second programming voltage node.
The third programming transistor of each odd numbered user-programmable resistor module is connected between its input circuit node and the first programming voltage node. The third programming transistor of each even numbered user-programmable resistor module is connected between its input circuit node and the second programming voltage node.
As is the case where a single user-programmable resistor module is used according to the present invention, a termination programming transistor is connected between the output circuit node of the last user-programmable resistor module in the plurality of series-connected user-programmable resistor modules and a programming voltage node. The second programming voltage node is used where the last user-programmable resistor module is an odd numbered one and the first programming voltage node is used where the last user-programmable resistor module is an even numbered one.
According to the present invention, the resistive element of any user-programmable resistor module is used by programming the first and second antifuses to connect the resistive element between the first and second circuit nodes and leaving the third and fourth antifuses unprogrammed. Similarly, the resistive element of any user-programmable resistor module is unused by programming the third and fourth antifuses to connect the first and second circuit nodes together and by leaving the first and second antifuses unprogrammed.
When a plurality of series-connected user-programmable resistor modules are used in accordance with the present invention, persons of ordinary skill in the art will appreciate that different combinations of resistor values such as an R2R ladder may be used to increase its versatility.